1. Field of the Invention
This invention relates to the molecular cloning, purification, characterization and inhibition of farnesyl:protein transferase, an enzyme involved in expression of the cancer phenotype, for example, in the transfer of farnesyl groups to oncogenic ras proteins. In particular aspects, the invention relates to nucleic acid segments encoding mammalian enzyme subunits which can be used as probes for the selection of related sequences or in the production of the holoenzyme or subunit polypeptides thereof, to the purification of the native or recombinant enzyme, as well as to assay methods for the identification of candidate substances which will inhibit the activity of the enzyme.
2. Description of the Related Art
In recent years, some progress has been made in the elucidation of cellular events lending to the development or progression of various types of cancers. A great amount of research has centered on identifying genes which are altered or mutated in cancer relative to normal cells. In fact, genetic research has led to the identification of a variety of gene families in which mutations can lead to the development of a wide variety of tumors. The ras gene family is a family of closely related genes that frequently contain mutations involved in many human tumors, including tumors of virtually every tumor group (see, e.g., Bos, 1989). In fact, altered ras genes are the most frequently identified oncogenes in human tumors (Barbacid, 1987).
The ras gene family comprises three genes, H-ras, K-ras and N-ras, which encode similar proteins with molecular weights of about 21,000 (Barbacid, 1987). These proteins, often termed p21.sup.ras, comprise a family of GTP-binding and hydrolyzing proteins that regulate cell growth when bound to the inner surface of the plasma membrane (Hancock, et al., 1989; Scheler et al., 1989). Overproduction of P21.sup.ras proteins or mutations that abolish their GTP-ase activity lead to uncontrolled cell division (Gibbs et al., 1989). However, the transforming activity of ras is dependent on the localization of the protein to membranes, a property thought to be conferred by the addition of farnesyl groups (Hancock et al., 1989; Casey et al., 1989).
A precedent for the covalent isoprenylation of proteins had been established about a decade ago when peptide mating factors secreted by several fungi were shown to contain a farnesyl group attached in thioether linkage to the C-terminal cysteine (Kamiya et al., 1978; 1979; Sakagami et al., 1981). Subsequent studies with the mating a-factor from Saccharomyces cerevisiae and farnesylated proteins from animal cells have clarified the mechanism of farnesylation. In each of these proteins the farnesylated cysteine is initially the fourth residue from the C terminus (Hancock, et al., 1989; Scheler et al., 1989; Gutierrez et al., 1989). Immediately after translation, in a sequence of events whose order is not yet totally established, a farnesyl group is attached to this cysteine, the protein is cleaved on the C-terminal side of this residue, and the free COOH group of the cysteine is methylated (Hancock et al., 1989; Gutierrez et al., 1989; Lowry et al., 1989; Clarke et al., 1988). All of these reactions are required for the secretion of active a-factor in Saccharomyces (Scheler et al., 1989).
Most, if not all, of the known p21.sup.ras proteins contain the cysteine prerequisite, which is processed by farnesylation, proteolysis and COOH-methylation, just as with the yeast mating factor (Hancock et al., 1989; Scheler et al., 1989; Gutierrez et al., 1989; Lowry et al., 1989; Clarke et al., 1988). The farnesylated p21.sup.ras binds loosely to the plasma membrane, from which most of it can be released with salt (Hancock, et al., 1989). After binding to the membrane, some P21.sup.ras proteins are further modified by the addition of palmitate in thioester linkage to cysteines near the farnesylated C-terminal cysteine (Hancock et al., 1989). Palmitylation renders the protein even more hydrophobic and anchors it more tightly to the plasma membrane.
However, although it appears to be clear that farnesylation is a key event in ras-related cancer development, prior to now, the nature of this event has remained obscure. Nothing has been known previously, for example, of the nature of the enzyme or enzymes which may be involved in ras tumorigenesis or required by the tumor cell to achieve farnesylation. If the mechanisms that underlie farnesylation of cancer-related proteins such as P21.sup.ras could be elucidated, then procedures and perhaps even pharmacologic agents could be developed in an attempt to control or inhibit expression of the oncogenic phenotype in a wide variety of cancers. It goes without saying that such discoveries would be of pioneering proportions in cancer therapy.